Chalk is made up of fine particles of calcium carbonate (sometimes calcium sulphate) compacted to the form of a rod. This is a porous material containing a large number of pores of varying sizes, which are filled with air when the chalk is dry. Further, these pores are interconnected. When a piece of chalk is dipped into water, the fine pores suck water due to capillary action as a consequence of surface tension at the junction of the calcium carbonate particles and water. At the same time, the relatively larger pores do not suck water as efficiently. Rather, the air displaced from the fine pores gets released from the 'larger' pores opening to the surface of the chalk. This air comes out as bubbles. When all the pores are saturated with water, no more bubbles come out.This might appear as if a chemical reaction between the chalk and water takes place producing a gaseous product, which bubbles out. But it is not so. If it was a chemical reaction, then it should not have stopped bubbling before the chalk has been 'used up' completely in the reaction. Further, a simple test is recommended to confirm this. Dry the soaked chalk completely. Now, again if it is dipped into water it would start bubbling indicating the validity of the above explanation.
